Cleavable diepoxide for removable epoxy compositions

ABSTRACT

A cleavable epoxy resin composition suitable for encapsulating electronic chips and a method for making the composition comprises the cured reaction product of a diepoxide; a cyclic dicarboxylic anhydride curing agent mixture; a 1,3-diaza compound having two nitrogen atoms present with one nitrogen atom doubly bonded to the central carbon and singly bonded to one other carbon, and the other nitrogen atom singly bonded to the central carbon and singly bonded to another carbon and singly bonded to a hydrogen. The 1,3-diaza compound serves either as the sole catalyst or in combination with a tertiary amine catalyst different from the diaza compound. The composition may include an optional hydroxy functional compound capable of reacting with the cyclic anhydrides to form a half ester thereby initiating the reaction between the diepoxide and the cyclic dicarboxylic anhydride curing agent. The resin can be used for the encapsulation of electronic parts, but can be removed by a solvent. This feature allows electronic components to be recycled.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is related to U.S. Ser. No. 08/210,879 filedMar. 18, 1994, entitled "Cleavable Diepoxide for Removable EpoxyCompositions" which is hereby incorporated by reference and which hasbeen assigned to the assignee herein.

FIELD OF THE INVENTION

The present invention generally relates to a chemical compound to beused as an epoxy. More particularly, the invention relates to acleavable epoxy resin composition which can be used as coatings,adhesives, structural components, and encapsulants for electronic chipsmounted onto substrates. Because the epoxy is cleavable, individualcomponents that have been encapuslated with the epoxy can be replaced.

BACKGROUND OF THE INVENTION

Epoxy resins are thermoset plastics for coatings, adhesives, structuralmaterials, electrical insulation, encapsulants, etc. Epoxy resins areparticularly well suited to protect electronic devices in packaging.Epoxies are applied in unreacted or partly reacted form, which meansthat the viscosity of the material can be quite low, providing ease ofprocessing and good wetting of device surfaces. Curing of the materialat some moderate temperature then generates the rigid epoxy matrixdesired for device protection. Filler additives are generally includedin the matrix to reduce the coefficient of thermal expansion (CTE) ofthe cured material to minimize stresses induced by the difference inexpansion of the plastic and the device during thermal cycling. Asdescribed in Lee, H., and Neville, K., Handbook of Epoxy Resins, McGrawHill (1967), fully cured epoxy resins, with or without filler, areheavily crosslinked insoluble network polymers. As thermosettingcompositions, the epoxy materials are difficult or impossible to removeafter curing. As a result, removal of such epoxy materials fromelectronic packages without damage to the circuitry or devices has beenand is virtually impossible.

Epoxy encapsulants are particularly valuable for reinforcement of solderjoints against thermal fatigue and encapsulation of wire bonded chips.For these applications, stability to ambient moisture is essentialbecause degradation of the encapsulant cannot be tolerated. A severelimitation of the epoxy reinforcement for the solder joints andencapsulation of wire bonded chips, however, is the fact that curedepoxy resins are insoluble and infusible, which means that thereinforced solder joints and wire bonds cannot be reworked. Theinability to replace one defective component on a microelectronicassembly renders all the other valuable components on that assemblyuseless, thus, the non-reworkability of conventional epoxy materials isa severe limitation on their applicability for solder reinforcement orencapsulation of wire bonds.

Another attribute of the epoxy thermosets is their intractability aftercuring. Curing converts the epoxy thermosets from low molecular weightprecursors to a network polymer of essentially infinite molecularweight. Previously considered an asset, the intractability of thermosetshas become a liability because of concerns about their longevity in theenvironment. Many manufacturers are either voluntarily or by governmentregulation taking responsibility for disposing or recycling theirproducts. Intractable thermosets are not compatible with the concept ofdesign for disassembly and recycling, whether the epoxies are used asstructural components, adhesives, or encapsulants. As demand increasesfor recyclable products, thermosets designed for disassembly on themolecular scale having cleavable diepoxide materials may well offer ameans of maintaining the utility of thermoset materials.

U.S. Pat. No. 3,023,174 to Batzer et al., owned by the assignee herein,and British Patent 865,340 disclose compositions based on diepoxideshaving linear ketal or acetal linkages. There is no mention of thepossible utility of such diepoxides with respect to cleavability indilute acid as disclosed in the present invention. In fact, stability ofthe cured ketal diepoxide in strong acid is specifically mentioned. U.S.Pat. No. 2,896,462 discloses diepoxides containing cyclic acetal groups,which although expected to be sensitive to degradation by acids, are infact surprisingly resistant to acids.

U.S. Pat. No. 4,153,586 refers to reaction products of epoxides whichare not suitable for curing to rigid matrices by reaction withcrosslinking agents such as cyclic anhydrides. Although this patentdiscloses ketal and acetal diepoxides there is no mention ofcleavability of the epoxides or their utility as a removable cured epoxymaterial.

U.S. Pat. Nos. 3,759,954; 3,879,422 and 3,956,317 all disclosecompositions of matter covering diepoxides containing one or more cyclicacetals and ketals but there is no mention of cleavability of theepoxides nor of their utility in epoxy compositions that are removableafter curing. The compounds are cured to epoxy matrices which are notcleavable.

U.S. Pat. No. 4,159,221 discloses a method to hermetically seal anelectronic circuit package. The reference discloses that the sealantused is an epoxy which is asserted to be readily soluble after beingcured. A partially cured epoxy, such as is disclosed in the reference,may be soluble but will not possess the physical properties needed toreinforce solder interconnections and/or wirebonds.

SUMMARY OF THE INVENTION

Thus, there exists a need which has been provided by the presentinvention for the combination of a high viscosity of an epoxy precursormixture, a high modulus, and a low CTE of the cured matrix which hasbeen found to be advantageous for an important device protectionapplication. A particularly useful embodiment is the protection ofwirebond interconnections of chips to ceramic, epoxy-glass or othersubstrates. The high viscosity precursor mixture efficiently andcompletely encapsulates the wirebonds and envelopes all interconnectionswith epoxy but prevents the mixture from flowing off the wirebonds ontothe substrate. Cured epoxy generally provides protection of thewirebonds from physical and environmental damage. The epoxy is removableto allow rework of a single chip in a microelectronic assembly so wideapplication of this encapsulation method is expected.

The present invention incorporates a cleavable link in the diepoxidemonomer which allows the thermoset network to be broken down in specialsolvents. This improvement allows the epoxy to be removed for repair,replacement, recovery, or recycling of the article of which the epoxy isa part. For example, in encapsulation of electronic chips, testing thechips to ascertain that the product satisfies the manufacturingspecification and, if needed, removing the chips for rework bydissolving the epoxy without destroying the chip or substrate.

The solvent system used to remove the cured diepoxide is another aspectof the present invention. While the solvent system dissolves the curedcleavable diepoxide compositions, it does not degrade other materialssuch as the copper wiring or the insulating dielectric material ofprinted circuit cards. The solvent systems used in accordance with thepresent invention provide an epoxy removal process compatible withmanufacturing and environmental concerns.

Thus, the present invention discloses a means of achieving a crosslinkedepoxy matrix maintaining all the advantages of previously known epoxymaterials that is also easily removable if the need arises. Pursuant tothe present invention, a straightforward synthesis is carried out toprepare a diepoxide having linear ketal or acetal linkages. Theinvention thus comprises a cured diepoxide composition and a method forsynthesizing the composition, the composition of which is capable ofbeing readily cleaved and removed in acid-containing solvents,comprising the reaction product of: a diepoxide in which an organiclinking moiety is the connection between the two epoxy groups of thediepoxide includes an acyclic acetal group; a cyclic dicarboxylicanhydride curing agent mixture of cyclic dicarboxylic anhydride curingagents present at a concentration such that the anhydride/diepoxideratio of equivalents is less than or equal to 0.90; a 1,3-diaza compoundhaving two nitrogen atoms present with one nitrogen atom doubly bondedto the central carbon and singly bonded to one other carbon, and theother nitrogen atom singly bonded to the central carbon and singlybonded to another carbon and singly bonded to a hydrogen, in which the1,3-diaza compound serves as either the sole catalyst or in combinationwith a tertiary amine catalyst which is different from said diazacompound. The invention also comprises a method of coating, protecting,encapsulating, reinforcing, assembling, or fabricating a device, anarticle of commerce or a chemical product with a cured diepoxidecomposition which is capable of undergoing controlled degradation in theenvironment or of being readily cleaved and removed in solvents, whereinthe epoxy composition comprises the reaction product of: a diepoxide inwhich the connection between the two epoxy groups of the diepoxideincludes a cleavable linear acetal group; a cyclic dicarboxylicanhydride curing agent; a catalyst; and a hydroxy functional initiator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the generalized structure of diepoxide containing linear ketalor acetal links.

FIG. 2 is the preferred structure of diepoxides containing linear acetaland ketal links.

FIG. 3 depicts various diepoxides that can be used in accordance withthe present invention.

FIG. 4 depicts various 1,3-diaza compounds be used as catalysts in theinvention.

DESCRIPTION OF THE INVENTION

Cycloaliphatic epoxides are a class of epoxy resins particularly usefulfor a variety of applications including electrical insulation, potting,encapsulation, coatings, etc. They are generally formulated with acyclic anhydride curing agent such as hexahydrophthalic anhydride(HHPA), which reacts with the epoxy in the presence of an amine catalystto form a thermoset network during thermal curing. Because this networkextends to macro-scale dimensions, it is insoluble and infusible, i.e.intractable.

The present invention is based on the recognition that theintractability of the cured epoxy network may result from thetetrafunctional nature of the diepoxide starting material in itsreaction with the difunctional cyclic anhydride. If the epoxide was onlydifunctional as would be the case with a monoepoxide, reaction with thedifunctional anhydride could only form a linear polymer not a highlycrosslinked network polymer. Thus, the link between the two epoxy groupsof the diepoxide is responsible for the network formation; and cleavageof such links would convert the network immediately to a collection ofsmall molecules, which would be soluble. The diepoxide forms a networkof macro-scale dimensions with a difunctional hardener such as a cyclicanhydride. Without degradation, such a network cannot dissolve in anysolvent. If the two epoxy groups of the diepoxide can be connected by acleavable bond or link, then cleavage of such links would convert thenetwork immediately to a collection of small molecules, which will besoluble.

In searching for appropriate structures that can serve as links for thediepoxide, the following criteria are important: (1) the link should bestable under conditions to which the cured matrix would normally beexposed; (2) the link should be sufficiently stable to permit thenetwork to perform its function in a specific application; (3) the linkshould be readily cleaved under specific conditions; (4) the link shouldbe unreactive in the curing reaction of the epoxy matrix; and (5) apractical synthesis of the diepoxide containing the link should beavailable.

The ketal and acetal groups have been identified as candidates meetingthe above criteria For the purposes of this invention, the term "acetal"refers to the 1,1-dialkoxy group as depicted in FIGS. 1-3 where R and R'can be alkyl, aryl, aralkyl or hydrogen. The general use of the term"acetal" includes ketals where R and R' is equivalent to alkyl, aryl, oraralkyl; acetals where R is equivalent to alkyl, aryl or aralkyl andR'=H and formals where R and R'=H.

As disclosed in March, J. Advanced Organic Chemistry (3d ed.), WileyInterscience 329-331 (1985), the known organic chemistry of ketals andacetals indicates that they are exceedingly stable to hydrolysis in theabsence of acids, but break down readily in acid, even weak acids.Ketals and acetals are not subject to reactions similar to those ofepoxy groups, and thus an acetal or a ketal link should not be affectedby the curing reaction of the epoxy matrix. Acetals can be hydrolyzed inacidic aqueous solutions, but they are also susceptible totrans-etherification under acidic conditions. Because the networkfragments are organic solvent-soluble and not water-soluble, it has beenfound that the best solvents for dissolution of the cleavable networksare those containing an alcohol and some organic acid such asmethanesulfonic acid or p-toluenesulfonic acid. It then becomes possibleto use an alcohol as both the solvent and the reactant which eliminatesthe necessity of adding water.

Control of the degradability/stability of the compositions with respectto ambient moisture is achieved, pursuant to the present invention, byusing three variables in the formulation. First, the structure of thecleavable link can be varied to adjust the stability of the link tohydrolysis. The rate of hydrolysis of acetals is affected by thesubstituents on the central carbon of the acetal. Considering forillustration purposes only methyl and hydrogen substituents, the formal,with two hydrogens on the central carbon is slowest to hydrolyze; theacetal, with one hydrogen and one methyl, hydrolyzes considerably moreeasily; and the ketal, with two methyls, hydrolyzes the fastest of thethree. By choosing diepoxides linked by formal, acetal, and ketalgroups, or by choosing some mixture of these; the formulator can adjustthe degradability of the resulting thermoset network to match therequirements of his application. For some applications, addition ofconventional, non-cleavable diepoxides such as 3,4-epoxycyclohexylmethyl3,4-epoxycyclohexanecarboxylate or bis-phenol A diglycidyl ether, may beadvantageous.

The second variable in the formulation controlling stability is theratio of cyclic anhydride to epoxy. If this ratio, on an equivalencebasis, is less than 0.90, the stability against moisture is markedlyenhanced. After the curing reaction is complete, any unreacted anhydridewill readily hydrolyze in the presence of moisture to generatecarboxylic acid. This acid in the network will catalyze the cleavage ofthe acetal links, leading to relatively rapid degradation of the networkby moisture. By using an excess of the diepoxide, all traces ofanhydride are reacted, leaving none to generate acid and catalyze thedegradation.

The third variable in the formulation controlling the stability is theamount of the 1,3-diaza compound used as a catalyst, as measured by itsratio to the epoxy in the formulation. The stability against moisture ismarkedly enhanced if ratio is less than 0.033, on an equivalence, basis.

Lowering the anhydride level in the cycloaliphatic epoxide formulationmeans that the mixture is out of stoichiometric balance, with excessepoxy groups. Without any further changes, the network will be lower incrosslink density and hence considerably lower in glass transitiontemperature (Tg). For applications which require both high moisturestability and a high Tg, such as for example, microelectronicencapsulation or solder joint reinforcement, a means of raising the Tgof formulations containing an anhydride/epoxy ratio of less than 0.90 isneeded.

Certain 1,3-diaza compounds arc especially effective in promotingepoxy--epoxy reactions, thereby increasing the crosslink density and Tgof formulations with anhydride/epoxy ratios of less than 0.90. In U.S.patent application Ser. No. 08/210,879, 1,3-diaza compounds connectingthe two nitrogens and in which the second nitrogen is a tertiary aminewere disclosed to raise the Tg of cured epoxy compositions with lessthan stoichiometric amounts of anhydride. The increase in Tg of twentyto thirty degrees Celsius in comparison to similar compositionscontaining conventional tertiary amine catalysts was noted. Thecompositions disclosed in the Ser. No. 08/210,879 application were1,3-diaza compounds in which neither nitrogen was bonded to hydrogen,because it was believed that hydrogen bound to nitrogen in suchcompounds would inhibit their activity in promoting epoxy--epoxyreactions and raise the Tg. But 1,3-diaza compounds in which onenitrogen is doubly bonded to the carbon connecting the two nitrogens andthe second nitrogen is a secondary amine, i.e., bonded to one hydrogen,are even more effective at catalyzing epoxy--epoxy reactions and raisingthe Tg. Increases in Tg of ten to twenty degrees Celsius have beenobserved using the 1,3-diaza compounds of the present invention comparedto the 1,3-diaza compounds previously disclosed. For many applications,including especially microelectronic encapsulation, higher Tg isparticularly advantageous in order to provide the mechanical propertiesneeded. The compounds effective as catalysts in raising the Tg offormulations containing anhydride/epoxy ratios less than 0.9 are those1,3-diaza compounds in which one nitrogen is doubly bonded to the carbonconnecting the two nitrogens and in which the second nitrogen is asecondary or tertiary amine. The 1,3-diaza structure can be part of acyclic or a bicyclic structure. Examples include imidazole;2-ethyl-4-methylimidazole; 2-methylimidazole; 5-methylimidazole;4-methylimidazole; 2-ethylimidazole.

The invention described herein is the family of epoxy compositions whichare removable in certain solvents because of the cleavable acetal linkconnecting the epoxy groups and which have high Tg because of the use ofcertain 1,3-diaza compounds as catalysts. The utility of thesecompositions depends on both their cleavability and their high Tg.

The ketal and acetal diepoxides of the present invention are synthesizedand then mixed with a cyclic anhydride, an amine promoter, and,optionally, a hydroxy functional initiator, a flexibilizer and/or aninorganic filler. The ketal/acetal diepoxide structure is shown in FIG.1, in which R and R' can be any combination of hydrogen, methyl, ethyl,propyl, iso-propyl, butyl, iso-butyl, other alkyl, phenyl, benzyl,substituted phenyl or substituted benzyl. Substituents on the phenyl orbenzyl can be at one or more of the available aromatic positions and canbe a halogen such as chlorine, bromine, or fluorine, a nitro group, anyalkyl group such as methyl, ethyl, or isopropyl, an alkoxy group such asmethoxy, ethoxy, or isopropoxy, an acyl such as acetyl or benzoyl, orany of the family of aromatic substituent groups well known in organicchemistry. In addition to the structure of FIG. 1, the diepoxide can beany diepoxide in which the two epoxide groups are connected by an acetalor ketal as in FIG. 2, in which R and R' are as in FIG. 1. The preferreddiepoxide structure is acetaldehyde bis-(3,4-epoxycyclohexylmethyl)acetal (more conveniently known as acetal diepoxide).

The epoxy structures suitable for use in this invention are thosederived from olefinic alcohols. The alcohol functionality preferably isan aliphatic primary or secondary alcohol group, most preferably aprimary alcohol group. The olefinic functionality is preferably analiphatic double bond, most preferably a mono-substituted or a1,2-disubstituted double bond, and must be suitable for epoxidation withan epoxidation reagent known in the art, such as peracetic acid,perbenzoic acid, metachloroperbenzoic acid, potassium peroxymonosulfate,and the like. Alternative olefinic alcohols and the acetal or ketaldiepoxides derived from them are shown in FIG. 3, in which R and R' areas in FIG. 1.

The cyclic anhydride can be any of the well known anhydride curingagents, see, e.g., H. Lee and K. Neville, Handbook of Epoxy Resins,McGraw-Hill, Chapter 12 (1967), including hexahydrophthalic anhydride,methyl-hexahydrophthalic anhydride, nadic methyl anhydride, and maleicanhydride, etc. For microelectronic applications, the preferredcompositions will have anhydride/epoxy equivalence ratios of 0.90 orless to minimize degradation by moisture.

The hydroxy functional initiator can be any high boiling alcohol orpolyol, such as ethylene glycol, diethylene glycol or the like.Optionally the flexibilizer can serve as the hydroxy functionalinitiator, by using a polyether diol such as polyethylene glycol,polypropylene glycol, poly(caprolactone)diol or poly(oxybutylene)diol.When using an imidazole catalyst with a secondary hydrogen, it has beenfound that a hydroxy functional initiator does not have to be used toobtain excellent crosslinking.

The amine promoter can be any tertiary amine or aromatic amine such asbenzyldimethylamine, triethylamine, pyridine, imidazole, propoxylatedimidazole, 1,4-diazabicyclo 2.2.2!octane, and the like. Most preferably,for microelectronic applications, the preferred amine promoters arethose which will raise the Tg of the moisture-stable formulations, forexample, the 1,3-diaza compounds of FIG. 4.

Encapsulants are commonly filled with an inorganic powder to reduce thecoefficient of thermal expansion. The optional inorganic filler can beany of the powdered solids known in the art, such as alumina, silica,zinc oxide, talc, etc. For microelectronic applications, the preferredfiller is a highly purified form of silica with particle size of 25microns or less. The amount of filler may vary but is preferred in therange 50-72% on a weight basis.

The optional flexibilizer can be any of the well known materials usedfor this purpose such as the Union Carbide ERL-4350, LHT-34 or LHT-240or the B. F. Goodrich butadiene-acrylonitrile copolymers sold under thetradename HYCAR. In addition, any polyetherdiol or polyesterdiol can beused as a flexibilizer including polyethylene glycol, polypropyleneglycol, poly(caprolactone)diol, or poly(oxybutylene)diol. A preferredflexibilizer is one which does not significantly depress the glasstransition temperature of the formulation, such as the maleic anhydrideadducts of polybutadiene resins sold by Ricon Resins as R-130.

The epoxy formulations are cured by heating at 90-200° C. for one to sixhours, preferably 100-150° C. for about two hours to form a hardtack-free solid. The preferred curing schedule includes a gel cure forabout one hour at 80-100° C., followed by a post-cure of about two hoursat 135-165° C. It is important that the cured resin resulting from thecleavable diepoxide not be adversely affected by the environment towhich it is likely to be exposed. Under the long-term exposure toelevated temperature and humidity (85° C./85% RH), the ketal diepoxideformulation showed significant softening. The corresponding acetaldiepoxide (FIG. 1, R=methyl, R'=hydrogen) formulation was unchangedafter four weeks in elevated temperature and humidity testing.

After curing, the cleavable diepoxide formulations can be dissolved byhydrolysis or transetherification of the cleavable link, which is anacetal group connecting two cycloaliphatic epoxy groups. Acetals andketals are generally easily cleaved in aqueous acid, but in order todissolve the matrix an organic solvent is also needed. Many mixtures oforganic solvents, acid or acids, and water can be used. For the purposesof this invention, suitable acids include organic acids such as aceticacid, propionic acid, chloroacetic acid, benzoic acid and the like;sulfonic acids such as benzenesulfonic acid, p-toluenesulfonic acid,methanesulfonic acid and the like; inorganic acids such as sulfuricacid, phosphoric acid, hydrochloric acid, and the like; and Lewis acidssuch as boron trifluoride etherate, aluminum chloride, stannic chlorideand the like. These structures are exemplary only and are disclosed toillustrate the type of solvents and acids to be used. The preferredacids are methanesulfonic acid and p-toluenesulfonic acid. The followingmixtures are given only as examples.

The temperature of the solvent mixture can be 25° C. or above, but formost rapid dissolution the solvents should be heated to boiling or nearboiling. One useful mixture is that of ethanol, acetic acid, and waterwhich is particularly effective in dissolving the cured formulationsbased on the ketal diepoxide. Other suitable solvent mixtures include acombination of gamma-butyrolactone, water, and phosphoric acid and acombination of butanol, acetic acid and water. Acetals and ketals arealso susceptible to trans-etherification under acidic conditions. Thusit becomes possible to use an alcohol as both the solvent and thereactant, removing the necessity of adding water to the system therebyreducing the likelihood of corrosion of metallic components of thedevice. For example, a mixture of ethanol and an organic acid such asbenzenesulfonic acid, para-toluenesulfonic acid, or methanesulfonic acidcan be used to dissolve the cured epoxy based on the acetal diepoxide.Transetherification using a primary alcohol such as ethanol and anorganic acid such as methanesulfonic acid is faster than hydrolysis inaqueous acid. An even faster dissolution rate was obtained by theincorporation of a portion of a less polar organic solvent such asxylene or benzyl alcohol or by the use of trifluoroethanol instead ofethanol.

The solvent used in accordance with the present invention comprises10-100 parts, preferably about thirty parts, of a primary alcohol asexemplified by: ethanol, methanol, n-butanol, and n-propanol; 0-90parts, preferably about thirty parts, of a less polar organic solvent asexemplified by: benzyl alcohol, xylene, toluene; and one to ten parts,preferably about three parts of an organic acid as exemplified by:methanesulfonic acid, p-toluenesulfonic acid andtrifluoromethanesulfonic acid. Additionally, about thirty parts ofethylene glycol is used in the solvent. Optionally the solvent includesa surfactant and/or a corrosion inhibitor.

Even at refluxing temperatures, the alcohols are also very benign withrespect to attack of the dielectric matrix used in circuitry such as FR4epoxy. For example, a mixture of ethanol and an organic acid such asbenzenesulfonic acid, para-toluenesulfonic acid, or methanesulfonic acidcan be used to dissolve the cured epoxy based on the acetal diepoxide.

EXAMPLE 1

Starting with the preparations described as Examples 3, 4 of U.S. patentapplication Ser. No. 08/210,879 filed Mar. 18, 1994, this exampleprovides curing studies. The Union Carbide cycloaliphatic diepoxideERL-4221 (1.00 gram measured epoxy equivalent weight 70.5) was mixedwith melted hexahydrophthalic anhydride (0.700 gram) and imidazole(0.0175 gram). After thoroughly mixing the three components, silicapowder (NYACOL, 3.2 gram, 65% by weight) was blended into the mixtureuntil the mixture was smooth. It was dispensed into two teflon bar moldscontaining six 40×10×1.5 mm cavities. One set of samples was cured at150° C. for three hours and the second set was gelled at 80° C. for onehour and then cured at 150° C. for two hours. The bars were hardtack-free solids, as expected for a fully cured epoxy resin. A dynamicmechanical analyzer (DMA) run exhibited a modulus peak with a maximum at164° C. for the gelled sample and 175° C. for the cured sample.

The acetal diepoxide (1.00 gram) was also mixed with the anhydride (0.07gram), imidazole (0.0175 gram) and silica. The curing behavior was quitesimilar, hard and tack-free with a modulus peak at 140° C. for thesample cured at 150° C. for three hours.

EXAMPLE 2

Formulations based on acetal diepoxide with anhydride/epoxy ratios lessthan 0.95 have low Tg's using benzyldimethyl amine as catalyst.Imidazole catalysts, known to catalyze epoxy--epoxy reactions resultingin high Tg's (see, Farkas, A. and Strohm, P. F., 12 J APPL. POLYMER SCI.159-168 (1968)), were used to make cleavable epoxies.

Example 2A. Acetal diepoxide, 100 parts by weight, was mixed with 70parts of pre-melted hexahydrophthalic anhydride (HHPAanhydride/epoxy=0.66) and 7 parts imidazole (imidazole/epoxy=0.13). Tothe clear solution was added 65% by weight of silica powder and theformulation was thoroughly mixed. The homogeneous mixture was loadedinto a teflon bar mold containing six 40×10×1.5 mm cavities. The sampleswere cured at 150° C. for three hours.

Example 2B. A sample was prepared by exactly the same procedure as 2Aexcept 1.75 parts of imidazole (imidazole/epoxy=0.033) was used.

Example 2C. A sample was prepared by exactly the same procedure as 2Bexcept 80 parts of acetal diepoxide and 20 parts of 3,4-epoxycyclomethyl3,4-epoxycyclohexanecarboxylate (Union Carbide ERL-4221) was usedinstead of the 100 parts acetal diepoxide.

Example 2D. A sample was prepared by exactly the same procedure in 2Cexcept 70 parts of acetal diepoxide and 30 parts of ERL 4221 were used.

Example 2E. A sample was prepared by the same procedure in 2D except theformulation was gelled at 80° C. for one hour and cured at 150° C. fortwo hours.

Example 2F. The commercial cycloaliphatic epoxide, 3,4-epoxycyclomethyl3,4-epoxycyclohexanecarboxylate (Union Carbide ERL-4221) was used toprepare a similar sample for comparative purposes. The composition was100 parts ERL-4221, 70 parts HHPA, 7 parts imidazole, 10 partsflexibilizer and 65% silica.

EXAMPLE 3

The moisture stability of the cured epoxy samples was investigated byplacing the samples into a constant temperature/humidity chamber at 85°C. and eighty-five percent relative humidity (T/H) for various times.After T/H exposure, the samples were monitored for evidence ofdegradation using dynamic mechanical analysis. The acetal samples of2A-2F were subjected to T/H. Comparison of mechanical properties wasmade to the unexposed samples by measuring flexural modulus vs.temperature for both exposed and unexposed samples.

After one week T/H, Sample 2A showed a marked drop in Tg from 174° C. to126° C., and an order of magnitude decrease in modulus above Tg. After24 hours at 100° C. under vacuum, the Tg did increase to 138° C., stillwell below the initial reading. This decrease is larger than what youwould expect for reversible plasticization by absorbed water.

Sample 2B in contrast showed only a 15° C. decrease, 140° C. to 125° C.,after one week T/H. However, after 24 hours at 100° C. under vacuum, theTg increased to 140° C., equal to its original value.

Sample 2C in which the acetal diepoxide was reduced and ERL 4221, anon-cleavable diepoxide added, exhibited a 20° C. decrease, 150° C. to130° C. after one week T/H. The Tg increased by 19° C. after heating 24hours at 100° C.

Sample 2D also showed a decrease of 19° C. in Tg after one week T/H,which also was reversible. In this case the plasticization caused aninitial decrease in Tg to 134° C. and on heating at 100° C. for 24 hoursthe Tg increased to 146° C.

Comparative sample 2F, which contained no cleavable diepoxides but hadthe large amount of imidazole, similar to Sample 2A, showed a 75° C.decrease in Tg after one week T/H. Heating at 100° C. for 24 hoursresulted in only a partial recovery of the Tg.

A design matrix showed that, by far, the significant factor in T/Hdegradation, when imidazole is used as a catalyst, is the level ofimidazole. If the imidazole/epoxy ratio is kept low, 0.033 in theseexamples, a Tg of 150° C. can be obtained with no mechanical degradationof the encapsulant.

EXAMPLE 4

Removal of the resins derived from the cleavable diepoxides wasaccomplished by exposing samples of the mixtures cured in Example 1 to anumber of solutions. Cured samples of the mixture containing the UnionCarbide ERL 4221 were also exposed at the same time as a control. Asolution containing a 1:1:1 volume ration of ethylene glycol, n-butylalcohol and xylene containing 0.3 M MSA (methane sulfonic acid) washeated to 105° C. The cured acetal diepoxide and the ERL-4221 wereimmersed in this solution while stirring with a magnetic stirrer. Aftersix minutes complete dissolution of the acetal diepoxide had occurred(about 35 mg/min). The control sample was unchanged.

Samples from Example 1 were also subjected to 0.3 M MSA in 1:1 ethyleneglycol and n-butanol at 85° C. The acetal diepoxide had a dissolutionrate of 29 mg/min. and the control was unchanged. In pure ethyleneglycol and 0.3 MSA, the rate of dissolution of the acetal diepoxide isabout 14 mg/min.

Samples 2A to 2E dissolved in four to six minutes in a solution of 0.3MMSA in equal volume amounts of n-butyl alcohol, ethylene glycol andxylene at a temperature of 105 degrees Celsius.

EXAMPLE 5

A formulation similar to that of Examples 2A to 2E were prepared exceptten parts of Polymer 35 flexibilizer was added. The Tg of the curedsamples was about 10° C. lower than the Tg for the samples as describedin Examples 2A to 2E. No change was observed after two 5 weeks T/H. Thesamples were also soluble in solvents of Example 4.

EXAMPLE 6

A formulation similar to that of Example 2B was prepared except silicafiller (fine particle size, PQ Corporation) was added to the level of65% by weight. Stability in T/H and dissolution in the various solventsdisclosed in Example 4 was essentially unchanged.

EXAMPLE 7

A formulation similar to that of Example 6 is prepared and charged intoplastic syringes. After degassing under vacuum, the syringes are frozenat -40° C. until use. After thawing to room temperature, the epoxy isdispensed under slight pressure through a needle to the periphery ofwire bonds attached to silicon chips to an epoxy printed circuit board.The board is preheated to about 80° C. to facilitate flow of the filledepoxy to completely surround the wire bonds. After the epoxy has beenapplied to all devices on the substrate and flow is complete, thesubstrate is transferred to an oven and the epoxy is cured at 80° C. forone hour and then 150° C. for two hours.

As necessary for replacement of defective or obsolete chips on thesubstrate, the epoxy is removed by immersing the substrate in a hotmixture of n-butanol, xylene, and ethylene glycol, and MSA preferablyexcluding air. Once all the epoxy is removed, the substrate is carefullyrinsed in pure isopropyl alcohol, again preferably excluding air.

EXAMPLE 8

Using an epoxy formulation similar to that of Example 7, chips directlymounted on a printed circuit card by flip chip attach or by wire bondsare encapsulated in globtop fashion. After curing as previously, theprinted circuit cards are ready for shipment.

For purposes of replacement of defective or obsolete components or tofacilitate removal of lead-containing solder prior to disposal of theprinted circuit assembly, the epoxy is easily removed as in Example 7.

EXAMPLE 9

During microanalysis of various mechanical and electrical devices, avariety of electronic devices are routinely mounted in an epoxy pottingcompound and then subjected to cutting and polishing to expose aparticular surface of the device for detailed optical and scanningelectron microscopy. Recovery of the part for further analysis of othersurfaces was virtually impossible with conventional epoxy pottingcompounds. Using the present invention technique, a formulation based onthe ketal or acetal diepoxide as described in Example 2A to 2E allowsremoval of the potting compound as described in Example 4.

What is claimed is:
 1. A cured diepoxide composition which is capable ofbeing readily cleaved and removed in acid-containing solvents,consisting essentially of the reaction product of: a diepoxide in whichan organic linking moiety is the connection between the two epoxy groupsof the diepoxide includes an acyclic acetal group; a cyclic dicarboxylicanhydride curing agent or mixture of cyclic dicarboxylic anhydridecuring agents present at a concentration such that theanhydride/diepoxide ratio of equivalents is less than or equal to 0.90;a 1,3-diaza compound having two nitrogen atoms present with one nitrogenatom doubly bonded to the central carbon and singly bonded to one othercarbon, and the other nitrogen atom singly bonded to the central carbonand singly bonded to another carbon and singly bonded to a hydrogen,said 1,3-diaza compound serving either as the sole catalyst or incombination with a tertiary amine catalyst which is different from saiddiaza compound.
 2. The diepoxide composition as defined in claim 1 inwhich up to fifty percent of the diepoxide compounds do not contain thecleavable linear acetal group.
 3. The diepoxide composition of claim 1in which said 1,3-diaza compound is selected from the group consistingof: imidazole; 2-ethyl-4-methylimidazole; 2-methylimidazole;5-methylimidazole; 4-methylimidazole; 2-ethylimidazole.
 4. The diepoxidecomposition of claim 1 in which the cyclic anhydride or mixture ofcyclic anhydrides are selected from the group consisting of:hexahydrophthalic anhydride, tetrahydrophthalic anhydride,methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride,dodecylsuccinic anhydride, nadic anhydride, nadic methyl anhydride,trimellitic anhydride, and maleic anhydride.
 5. The diepoxidecomposition defined in claim 1 which said acetal diepoxide isacetaldehyde bis-(3,4-cyclohexylmethyl) diepoxide.